1. Technical Field
The disclosed embodiments generally relate to the field of voltage regulating or control systems. More particularly, the disclosed embodiments relate to an improved tap changing method and system for power delivery.
2. Description of the Related Art
A tap changer is a device used to change the load voltage or phase angle of a power delivery system. Typically, the selection of a tap adjusts the number of turns used in one or more of a transformer's windings. Tap changers most commonly are used to permit the regulation of the output voltage of a transformer or step voltage regulator to a desired level.
Tap changing may occur either while the transformer is energized (i.e., under load) or while the transformer is not energized (i.e., offline). A mechanical switching assembly is typically used to accomplish tap changing under load (TCUL) in power transformers and step-voltage regulators. To accomplish a tap change, older design load tap changers (LTCs) simply interrupt the load current, which is sometimes more than 1,000 amperes, with the simple parting of contacts under oil. This practice continues today.
The interruption of a high load current can lead to an arc between the contacts. To avoid this arc and the consequential deleterious effects of contact burn and oil decomposition, which leads to early failure or the need for maintenance, newer tap changers include contacts that are immersed in oil with the inclusion of a vacuum switch. In these designs, the current is commutated to a path through the vacuum switch for the current interruption. An early description of such a design, which is still commonly used today, is found in H. A. Fohrhaltz, Load-Tap Changing with Vacuum Interrupters, IEEE Transactions on PAS, vol. PAS-86, No. 4, April 1967, pp. 422-428. Vacuum switch technologies usually require the use of bridging reactors. The bridging reactor is itself a transformer, of perhaps one quarter of the size of the main transformer. It significantly adds to the cost and weight of the total assembly. It typically will also add to total internal losses, the resulting heat having to be dissipated with additional tank cooling provisions.
Various attempts have been made to replace the vacuum switch in a LTC with power thyristors. Some of these attempts can be categorized as “solid-state” technology, and the rest can be categorized as “hybrid” technology. Solid state LTCs can be characterized by the elimination of any mechanical switching assembly. The motivation is very high speed operation, i.e., one to three cycles (typically less than 50 milliseconds), and the opportunity to span multiple tap steps in a single operation. This feature provides more speed than is typically required for run-of-the-mill power distribution transformer applications, which typically involve a 30 second intentional time delay. Thus, it often adds an unnecessary expense. In addition, reliability issues can be extensive since the thyristors must be continuously active. An illustrative early embodiment of a solid-state implementation is found in U.S. Pat. No. 3,195,038, issued Jul. 13, 1965 to Fry.
In contrast, “hybrid” technology includes both mechanical switches and solid-state components (e.g., thyristors). In these designs, mechanical switches accomplish the tap position selection, while the thyristors only assist during the actual tap change event, which will typically occur less than 40 times per 24 hours. Because the mechanical switch is doing the actual tap position selection, fewer thyristors are required. One implementation of a hybrid LTC is found in U.S. Pat. No. 4,363,060, issued Dec. 7, 1982 to Stich.
A problem with hybrid designs is that they involve the use of a means (usually a power resistor) to limit the magnitude of a current that may circulate in the electronic circuit during the tap change. Others have attempted to avoid such a need with the use of more complex (and expensive) circuitry and gate-turn-off thyristors. Prior hybrid systems also require multiple power switches, further adding to the cost of the circuit.
The disclosure contained herein describes attempts to address one or more of the problems described above.